I. Field of the Invention
The current invention relates to wireless telecommunications. More particularly, the present invention relates to a novel and improved transmitter design for enhancing the reliability of communications in a wireless communications system.
II. Description of the Related Art
The use of code division multiple access (CDMA) modulation techniques is one of several techniques for facilitating communications in which a large number of system users are present. Other multiple access communication system techniques, such as time division multiple access (TDMA), frequency division multiple access (FDMA) and AM modulation schemes such as amplitude companded single sideband (ACSSB) are known in the art. However, the spread spectrum modulation technique of CDMA has significant advantages over these other modulation techniques for multiple access communication systems.
The use of CDMA techniques in a multiple access communication system is disclosed in U.S. Pat. No. 4,901,307, entitled xe2x80x9cSPREAD SPECTRUM MULTIPLE ACCESS COMMUNICATION SYSTEM USING SATELLITE OR TERRESTRIAL REPEATERSxe2x80x9d, assigned to the assignee of the present invention and incorporated by reference herein. The use of CDMA techniques in a multiple access communication system is further disclosed in U.S. Pat. No. 5,103,459, entitled xe2x80x9cSYSTEM AND METHOD FOR GENERATING SIGNAL WAVEFORMS IN A CDMA CELLULAR TELEPHONE SYSTEMxe2x80x9d, and in U.S. Pat. No. 5,751,761, entitled xe2x80x9cSYSTEM AND METHOD FOR ORTHOGONAL SPREAD SPECTRUM SEQUENCE GENERATION IN VARIABLE DATA RATE SYSTEMSxe2x80x9d, both assigned to the assignee of the present invention and incorporated by reference herein. The use of CDMA searchers is disclosed in U.S. Pat. No. 5,764,687, entitled xe2x80x9cMOBILE DEMODULATOR ARCHITECTURE FOR A SPREAD SPECTRUM MULTIPLE ACCESS COMMUNICATION SYSTEMxe2x80x9d, assigned to the assignee of the present invention and incorporated by reference herein. Code division multiple access communications systems have been standardized in the United States in Telecommunications Industry Association TIA/EIA/IS-95-A, entitled xe2x80x9cMOBILE STATION-BASE STATION COMPATIBILITY STANDARD FOR DUAL-MODE WIDEBAND SPREAD SPECTRUM CELLULAR SYSTEMxe2x80x9d, hereafter referred to as IS-95 and incorporated by reference herein.
The CDMA waveform, by its inherent nature of being a wideband signal, offers a form of frequency diversity by spreading the signal energy over a wide bandwidth. Therefore, frequency selective fading affects only a small part of the CDMA signal bandwidth. Space or path diversity on the forward or reverse link is obtained by providing multiple signal paths through simultaneous links to or from a mobile user through two or more antennas, cell sectors or cell-sites. Furthermore, path diversity may be obtained by exploiting the multipath environment through spread spectrum processing by allowing a signal arriving with different propagation delays to be received and processed separately. Examples of the utilization of path diversity are illustrated in U.S. Pat. No. 5,101,501 entitled xe2x80x9cSOFT HANDOFF IN A CDMA CELLULAR TELEPHONE SYSTEMxe2x80x9d, and U.S. Pat. No. 5,109,390 entitled xe2x80x9cDIVERSITY RECEIVER IN A CDMA CELLULAR TELEPHONE SYSTEMxe2x80x9d, both assigned to the assignee of the present invention and incorporated by reference herein.
In the CDMA demodulator structure used in some IS-95 systems, the PN chip interval defines the minimum separation two paths must have in order to be combined. Before the distinct paths can be demodulated, the relative arrival times (or offsets) of the paths in the received signal must first be determined. The demodulator performs this function by xe2x80x9csearchingxe2x80x9d through a sequence of offsets and measuring the energy received at each offset. If the energy associated with a potential offset exceeds a certain threshold, a demodulation element, or xe2x80x9cfingerxe2x80x9d may be assigned to that offset. The signal present at that path offset can then be summed with the contributions of other fingers at their respective offsets.
A method and apparatus of finger assignment based on searcher and finger energy levels is disclosed in U.S. Pat. No. 5,490,165, entitled xe2x80x9cFINGER ASSIGNMENT IN A SYSTEM CAPABLE OF RECEIVING MULTIPLE SIGNALSxe2x80x9d, assigned to the assignee of the present invention and incorporated by reference herein. In the exemplary embodiment, the CDMA signals are transmitted in accordance with IS-95. An exemplary embodiment of the circuitry capable of demodulating IS-95 forward link signals is described in detail in U.S. Pat. No. 5,764,687, entitled xe2x80x9cMOBILE DEMODULATOR ARCHITECTURE FOR A SPREAD SPECTRUM MULTIPLE ACCESS SYSTEMxe2x80x9d, assigned to the assignee of the present invention and incorporated by reference herein. An exemplary embodiment of the circuitry capable of demodulating IS-95 reverse link signals is described in detail in U.S. Pat. No. 5,654,979, entitled xe2x80x9cCELL SITE DEMODULATOR ARCHITECTURE FOR A SPREAD SPECTRUM MULTIPLE ACCESS COMMUNICATION SYSTEM,xe2x80x9d assigned to the assignee of the present invention and incorporated by reference herein.
In the exemplary embodiment, the signals are complex PN spread as described in U.S. patent application Ser. No. 08/856,428, entitled xe2x80x9cREDUCED PEAK TO AVERAGE TRANSMIT POWER HIGH DATA RATE IN A CDMA WIRELESS COMMUNICATION SYSTEM,xe2x80x9d filed Apr. 9, 1996, assigned to the assignee of the present invention and incorporated by reference herein, and in accordance with the following equations:
I=Ixe2x80x2PNI+Qxe2x80x2PNQxe2x80x83xe2x80x83(4)
Q=Ixe2x80x2PNQxe2x88x92Qxe2x80x2PNI.xe2x80x83xe2x80x83(5)
where PNI and PNQ are distinct PN spreading codes and Ixe2x80x2 and Qxe2x80x2 are two channels being spread at the transmitter.
The International Telecommunications Union recently requested the submission of proposed methods for providing high rate data and high-quality speech services over wireless communication channels. A first of these proposals was issued by the Telecommunications Industry Association, entitled xe2x80x9cThe cdma2000 ITU-R RTT Candidate Submissionxe2x80x9d. A second of these proposals was issued by the European Telecommunications Standards Institute (ETSI), entitled xe2x80x9cThe ETSI UMTS Terrestrial Radio Access (UTRA) ITU-R RTT Candidate Submissionxe2x80x9d. And a third proposal was submitted by U.S. TG 8/1 entitled xe2x80x9cThe UWC-136 Candidate Submissionxe2x80x9d (referred to herein as EDGE). The contents of these submissions is public record and is well known in the art.
In addition to the aforementioned properties, CDMA""s broadband nature permits the demodulation of signals having traversed different propagation paths. In U.S. Pat. Nos. 5,280,472, 5,513,176, 5,553,011, assigned to the assignee of the present invention and incorporated by reference herein, the usage of multiple sets of distributed antennas is employed to deliberately provide multiple paths of propagation. In the just mentioned U.S. Patents, sets of antennas are fed by a common signal with only time delay processing to distinguish signals. The transmit output of the base station is fed to a string of antenna elements for example with a coaxial cable. The antenna elements connect to the cable using power splitters. The resulting signals, amplified and frequency converted as necessary, are fed to the antennas. The salient features of this distributed antenna concept are as follows: (1) simple and inexpensive dual antenna node design; (2) neighboring antennas have time delays inserted in feed structure so signals received and transmitted from neighboring antennas are distinguishable by PN temporal processing; (3) exploitation of direct sequence CDMA""s ability to discriminate against multipath; and (4) creation of deliberate multipath that satisfies discrimination criteria.
Antenna transmit diversity as well as multi-carrier transmission are promising new technologies that improve transmission resistance to fading by offering space and/or frequency diversity. In the antenna transmit diversity case for example, the data to be transmitted is encoded into symbols, which are then distributed among the antennas and transmitted.
Many techniques have been proposed for mitigating mutual interference between signals transmitted from the different antennas. Such techniques include delay transmit diversity, orthogonal transmit diversity (OTD), time switched transmit diversity (TSTD), time delayed transmit diversity (TDTD), and multi-carrier transmit diversity (MCTD). Each of these methods shares with the others a common goal of providing additional diversity in the transmitted signal through space, time, frequency or code space. These methods are known in the art and have been described in proposals to the International Telecommunications Union in response to their request for proposed Third Generation Wireless communication systems. Methods for introducing diversity into a transmitted signal are almost limitless by their very nature. Copending U.S. Pat. No. 6,215,777 entitled xe2x80x9cMethod and Apparatus for Transmitting and Receiving Data Multiplexed onto Multiple Code Channels, Frequencies and Base Stationsxe2x80x9d, filed Sep. 15, 1997, assigned to the assignee of the present invention and incorporated by reference herein, describes a matrix of methods for transmitting CDMA signals using multiple carriers and multiple code channels for introducing diversity into the transmitted signal.
In addition, the multi-carrier transmission, whether it uses antenna transmit diversity or not, must distribute the coded symbols among the different carriers, which is similar to distributing symbols among several antennas in an antenna transmit diversity system. One skilled in the art will appreciate that, in the case where a multi-carrier system uses a single transmit antenna, the channels utilizing the two carriers may still be thought of as independent transmission channels which may or may not suffer from correlated fading. Correlated fading is the phenomenon whereby each of the transmissions suffers degradation in a temporally correlated fashion.
In a system utilizing interleaving in conjunction with transmit diversity, it is desirable to fully utilize the gain offered by both techniques, as well as to make sure that the interleaver also performs well when the transmission channels become correlated. For example, in a system utilizing two transmission channels, using either two transmit antennas or two carriers, correlated fading in both transmission channels may cause the loss of adjacent transmitted coded symbols. Decoders such as trellis decoders and turbo decoders are often more susceptible to the loss of several successive symbols than to the loss of the same number of symbols spread throughout the data stream. In order to reduce the probability of loss of adjacent encoded symbols, interleavers such as block interleavers and turbo coded interleavers are employed. However, these traditional interleaving methods provide less time diversity when employed by traditional means in systems employing transmission diversity. Thus, there is a need felt in the art for a method of decreasing the chances of losing successive symbols in a system which utilizes transmit diversity.
The current invention enhances the performance of a system utilizing interleaving and transmit diversity by reordering the sequence of symbols transmitted along the different transmission channels. For example, in the case of a system using two transmission channels, the symbols being transmitted on one channel are shuffled with respect to the symbols transmitted through the other channel. This shuffling makes it less likely that successive symbols output by the interleaver are lost to correlated fading in the two transmission channels.
Suppose a source frame F is composed of N coded symbols Si (1 less than i less than N). Suppose also that these symbols Si are distributed over M transmitters (different carriers or antennas or both). The current invention splits the symbols into M groups Gj (1 less than j less than M), one for each transmitter. Then each of the groups, Gj, is interleaved independently.
A problem which can occur if the interleavers and splitter are not chosen correctly, or even worse if they are all identical, is that the performance will be severely degraded when the signals from the different transmitters go through channels that are correlated.
Consider the case where there are 2 transmitters that lead to 2 antennas to provide space diversity in a transmitted signal. If the symbols are allocated for transmission such that odd symbols are provided for transmission through the first antenna and the even symbols are provided for transmission through the second antenna and wherein the symbols are interleaved subsequent to their provision to the respective antennas. If traditional interleaving strategies are used, correlated fading will severely deteriorate performance.
The goal of the shuffle is to make sure that even if the different transmission paths from the different transmitters become correlated, the performance degradation is minor. One particularly efficient implementation of the shuffle that each shuffle cyclically rotates the symbols it receives. Here is an example:
Shuffler j: cyclically rotate the symbols to be transmitted by transmitter j by (jxe2x88x921)*N/M symbols. Thus, if N=4, M=2 and G2 after interleaving is xe2x80x9cabcdxe2x80x9d, then shuffler 2 would output xe2x80x9ccdabxe2x80x9d, which is xe2x80x9cabcdxe2x80x9d that has been cyclically rotated by N/M=2 symbols. An alternative embodiment of a shuffler is a flip. This transforms xe2x80x9cabcdxe2x80x9d into xe2x80x9cdcbaxe2x80x9d.
It will be understood by one skilled in the art that the shuffle operations, though presented as being subsequent to and separate from the traditional interleaving operation, in reality would probably be combined with interleaving, yielding a single operation in a real implementation.